This invention is directed to roll forming and more particularly, roll forming structural members.
Various products are formed by creating a skeleton formed of structural members and covering the skeleton with a "skin". For example, most modern aircraft, particularly those of a relatively large size, are formed by creating skeletons of structural members and covering the skeletons with sheets of aluminum alloy. The structural members are usually formed of an aluminum alloy or some other roll formable material. Many of the structures used to form such aircraft vary in cross-sectional size from one end to the other (e.g., a wing or a fuselage). Because of such cross-sectional variations and because the load carrying capability of such structures varies from region-to-region, obviously, the strength of the structural members can also vary. Further obviously, the structural efficiency of aircraft, particularly advanced aircraft, will be improved if the structural members used (e.g. ribs and stringers) can be made to vary in both thickness and cross section (height and/or width) without an undue increase in manufacturing costs, because such variations will allow a designer to vary the section modulus (e.g. strength) along the length of a structural member to suit loading conditions. The end result of this variation is to substantially reduce the weight and/or surface area of the resultant airframe, which reductions translate into improved range, speed and pay load.
Currently, aircraft structural members formed of aluminum alloy taper in thicknesses only, and not in height and width, except as noted below. Such members are formed in complex, variable orifice draw dies or in progressive rolls wherein the final roll stages are mechanically or hydraulically actuated so as to taper the thickness of the member being formed. It will be appreciated that structural members produced in this manner are severally limited as to the degree of variation in section modulus that can be produced. If it is desired to create an overall structural member that varies in cross section (as well as thickness), in the past, such members have been produced by first forming relatively short sections each having a fixed, but different cross-sectional height and/or width. The sections may or may not vary in thickness, as desired. These sections are spliced together using mechanical fasteners or bonded doublers to achieve the desired overall taper. Obviously, splicing is time consuming and expensive. Moreover, vibration and other movement of the resultant structure may result in a fatigue critical joint requiring substantial reinforcement. Hence, assembling tapered structural members in this manner has a variety of disadvantages.
The only presently available method of producing continuous, long variable cross-section structural members is to machine them from extrusions. According to this method, a blank that is large enough to encompass the total range of section taper is first extruded. The blank is then machined to the desired shape. It will be appreciated that this method is both costly and tedious. It is costly, first, because of the production time required and, second, because a large portion of the extrusion is lost in chips, which may or may not be recoverable. Even if recoverable, the chips must be reprocessed before they are usable. Since substantial amounts of energy are required to produce most metals, in particular aluminum, considerable energy waste is incurred in chip loss and the reprocessing of chips to a usable form.
Therefore, it is an object of this invention to provide a new and improved method of and apparatus for roll forming continuous tapered structural members.
It is a further object of this invention to provide a new and improved method of and apparatus for roll forming relatively long, continuous, tapered structural members suitable for use in forming the skeleton of an aircraft.
It is another object of this invention to provide a method of and an apparatus for roll forming tapered structural members that does not require that the members be machined to any significant degree after they have been formed.
It is yet another object of this invention to provide a method of and an apparatus for continuously forming structural shapes that vary both in thickness and taper (height and/or width).